In the field of semiconductor elements, such as transistors, IC chips, and LSI chips, plastic packages have now prevailed from the standpoint of cost and suitability to mass production. Epoxy resin composition have been of wide use as an encapsulating resin, achieving satisfactory results. In particular, an epoxy resin composition comprising an epoxy resin, a novolak phenolic resin as a hardener, a cure accelerator, and an inorganic filler such as silica powder has been made frequent use of for its excellent encapsulating properties.
In recent years, the competition for the cost of plastic packages has been heated up. As an approach to cost reduction, efforts have been made to reduce a molding cycle thereby to improve production efficiency. With this tendency, the demand for an encapsulating material with improved curing properties has been increasing, which has been met by increasing the amount of a cure accelerator to be added.
An epoxy resin composition previously added with a cure accelerator undergoes curing reaction while stored. In particular where added with a large quantity of a cure accelerator, the composition undergoes rapid reaction before use, having an extremely short pot life and reducing the working efficiency. Therefore, it has recently been proposed to use an epoxy resin composition containing microcapsules of a cure accelerator.
While excellent in storage stability, the epoxy resin composition containing encapsulated cure accelerator as a sole cure accelerating component has the following drawback. Because of the microcapsule shell to be melted and destroyed, a delay is produced in allowing the cure accelerator to act in molding the resin composition for semiconductor encapsulation. As a result, the composition is inferior in rapid curability. The disadvantage is particularly conspicuous when the molding temperature is low (usually 140 to 180.degree. C.).